[RFA] Code gen improvement for sim-decode.scm

[Ben Elliston]

   Are you convinced that nothing can possibly use that variable except
   ifield extraction?  Is the improvement to generated code merely the
   removal of a gcc warning?  (It should be able to zap an unused
   variable, resulting in no performance difference.)

Nothing else can use it because it's defined within block scope. Yes, the
improvement to generated code is to remove a GCC warning.

Ben